The present invention is in the field of photochromic glasses and particularly relates to surface-colored photochromic glass articles produced by a reduction heat treatment.
Silver halide-containing photochromic glasses are well known in the art, having been first described by Armistead et al. in U.S. Pat. No. 3,208,860, and thereafter finding commercial use principally in the manufacture of photochromic ophthalmic lenses. Such lenses darken upon exposure to actinic radiation, e.g. ultraviolet light, and fade in the absence thereof.
Second-generation silver halide-containing photochromic glasses exhibiting improved darkening and fading characteristics have also been recently introduced. One family of such glasses has been described by G. B. Hares et al. in a commonly assigned, copending application, Ser. No. 14,981, filed Feb. 28, 1979 U.S. Pat. No. 4,190,451.
Colored photochromic glasses, meaning photochromic glasses exhibiting non-neutral coloration in transmitted light in the undarkened state, are also known. These may be produced by the addition of light-absorbing colorants into the base glass, or by processing the glass during or subsequent to manufacture. U.S. Pat. Nos. 3,892,582 and 3,920,463 to Simms describe a post-manufacturing coloring treatment wherein pink, yellow or brown photochromic glasses are produced by firing silver halide-containing photochromic glass articles in a reducing atmosphere. U.S. Pat. No. 4,125,405 to Araujo et al. describes red silver halide-containing glasses, produced under reducing melting conditions, which are slightly photochromic.
The reduction-melted glasses reported by Araujo et al. are insufficiently photochromic for commercial use in photochromic products. Attempts to enhance the photochromic performance thereof by heat treatment typically destroy the red color of the glass. The yellow photochromic glasses described in the Simms patents are of commercial interest, but the pink and brown glasses are not, the brown color being too dark for most applications and the pink color being too light.
The yellow observed upon the heat treatment of photochromic glasses under reducing conditions in the manner disclosed by Simms is attributed to an absorption band caused by the precipitation of metallic silver in the glass during heat treatment. In silver-containing glasses free of other precipitated phases, the silver absorption band is manifested as an absorption peak centered at about 390 nm in the violet region of the spectrum. In the reduction-fired photochromic glasses reported in the aforementioned Simms patents, which contain precipitated silver halide in addition to the matrix glass, absorption peaks are reported in the blue region of the spectrum between about 430-460 nm.
The hue and intensity of the induced color in prior art glasses probably depended upon the position and intensity of the treatment-induced absorption peak. The deepest yellow colors were caused by strong absorption peaks at 430 to 460 nm, while the light pink color is now thought to have been caused by the same fundamental absorption peak as it first appeared in weak form at about 500 nm following a mild heat treatment.
While reduction firing treatments such as used in the prior art constituted a simple and convenient method for imparting surface coloration to photochromic glasses, they offered only a very limited range of color selection. Attempts to intensify the colors provided, for example by using more severe heat treatments, appeared merely to shift the fundamental absorption peak towards the violet, resulting in a more intensely yellow glass.